U.S. patent number 8,503,180 [Application Number 12/159,544] was granted by the patent office on 2013-08-06 for variable frequency drive system apparatus and method for reduced ground leakage current and transistor protection.
This patent grant is currently assigned to SMC Electrical Products, Inc.. The grantee listed for this patent is Geraldo Nojima. Invention is credited to Geraldo Nojima.
United States Patent |
8,503,180 |
Nojima |
August 6, 2013 |
Variable frequency drive system apparatus and method for reduced
ground leakage current and transistor protection
Abstract
As applications of variable frequency drives (VFD) (50) continue
to grow so do challenges to provide VFD (50) systems meeting
application specific requirements. For multiple reasons to include
safety standards and electromagnetic interference, reduced ground
leakage current is desirable. Building high output voltage VFDs
(50) using transistors rated at voltages lower than the VFD output
voltage is desireable for economic reasons. The apparatus and
method described herein meet these challenges and others, in part
by placing an electrically insulating plate (cp176) having high
thermal conductivity, a low dielectric constant, and high
dielectric strength between the heat sink plate of a VFD power
semiconductor module and a grounded cooling plate (80 TE). The
positive effects of this plate installation include reducing ground
leakage current induced by system capacitances to ground upon high
frequency voltage steps and increasing the effective dielectric
strength of the VFD's (50) transistor modules engaging in high
reliable VFD (50) voltage output for a given transistor rating.
Inventors: |
Nojima; Geraldo (Duluth,
GA) |
Applicant: |
Name |
City |
State |
Country |
Type |
Nojima; Geraldo |
Duluth |
GA |
US |
|
|
Assignee: |
SMC Electrical Products, Inc.
(Barboursville, WV)
|
Family
ID: |
38228524 |
Appl.
No.: |
12/159,544 |
Filed: |
December 30, 2005 |
PCT
Filed: |
December 30, 2005 |
PCT No.: |
PCT/US2005/047353 |
371(c)(1),(2),(4) Date: |
June 27, 2008 |
PCT
Pub. No.: |
WO2007/078285 |
PCT
Pub. Date: |
July 12, 2007 |
Prior Publication Data
|
|
|
|
Document
Identifier |
Publication Date |
|
US 20080303469 A1 |
Dec 11, 2008 |
|
Current U.S.
Class: |
361/707;
361/23 |
Current CPC
Class: |
H02P
27/06 (20130101); H02M 7/003 (20130101); H01L
2924/0002 (20130101); H01L 2924/0002 (20130101); H01L
2924/00 (20130101) |
Current International
Class: |
H02H
5/04 (20060101) |
Field of
Search: |
;361/43,44,47,719,753,799,762,767,23,707 ;324/551,509,545 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
Other References
Mitic. G. et al., "AlSiC Composite Materials in IGBT Power Modules"
Industry Applications Conference, 2000. Oct. 8, 2000, vol. 5. pp.
3021-3027. Piscataway, NJ, USA. XP0105 21717. cited by
applicant.
|
Primary Examiner: Nguyen; Danny
Attorney, Agent or Firm: Sughrue Mion, PLLC
Claims
What is claimed is:
1. A method of increasing dielectric strength of a power
semiconductor in a variable frequency drive (VFD) system
comprising: disposing an electrically insulating plate which has
high thermal conductivity, a low dielectric constant, and high
dielectric strength, an insulator and a heat sink between a VFD
power semiconductor module and a cooling plate; wherein an output
voltage of said VFD is greater than or equal to 690 V.
2. A variable frequency drive (VFD) system comprising: an inverter
bridge, of a VFD power semiconductor module, electrically connected
to the DC bus; an electrically insulating plate which has high
thermal conductivity, a low dielectric constant, and high
dielectric strength; and a cooling plate wherein the VFD power
semiconductor module is mounted on an insulator, a heat sink and on
the electrically insulating plate, having high thermal
conductivity, a low dielectric constant, and high dielectric
strenght; wherein the insulating plate is mounted on the cooling
plate; wherein the cooling plate is grounded and wherein an output
voltage of said VFD is greater than or equal to 690 V.
3. The VFD system according to claim 2, wherein the insulating
plate is a ceramic material.
4. The VFD system according to claim 2, wherein the insulating
plate is made of boron nitride.
5. The VFD system according to claim 2, wherein a capacitance is
formed across the electrically insulating plate between the VFD
power semiconductor module and the cooling plate, and wherein said
capacitance between the VFD power semoconductor module and the
cooling plate is less than inverter bridge transistor internal
capacitance to ground, and a total capacitance to ground of the VFD
system is reduced.
6. The VFD system according to claim 2, further comprising at least
one mounting device which enables electrical insulation and thermal
conduction between the VFD power semiconductor module and the
grounded cooling plate and secures the VFD power semiconductor to
the insulating plate and secures the insulating plate to the
grounded cooling plate.
7. A variable frequency drive (VFD) system comprising: a DC bus
which has a grounded neutral point; and VFD power semiconductor
modules, wherein transistor modules of an inverter bridge are
connected in series, the inverter bridges is electrically connected
to the DC bus, and an insulation of transistor modules is rated at
less than half of a full maximum DC bus voltage rating; an
electrically insulating plate which has high thermal conductivity,
a low dielectric constant, and high dielectric strength; and a
grounded cooling plate, wherein the VFD power semiconductor module
is mounted on an insulator, a heat sink and on the electrically
insulating plate, having high thermal conductivity, a low
dielectric constant, and high dielectric strength; wherein the
insulating plate is mounted on the grounded cooling plate; wherein
an effective insulation capability to ground of the VFD system is
increased, and wherein an output voltage of said VFD is greater
than or equal to 690 V.
8. The method according to claim 1, wherein said output voltage of
said VFD is greater than or equal to 4160 V.
9. The VFD system according to claim 2, wherein said output voltage
of said VFD is greater than or equal to 4160 V.
Description
BACKGROUND OF THE INVENTION
Conventional sinusoidal AC voltage supplies provide only fixed
motor speed and are unable to respond quickly to changing load
conditions. With the advent of variable frequency drives (VFDs), a
better performing motor at lower energy costs can be achieved. VFD
driven motors rapidly respond to changing load conditions, for
example in response to shock loads. VFD driven motors provide
precision torque output and continuous speed control, as well.
Because of their many advantages, the utilization of VFDs in
industrial applications continues to grow.
A conventional medium voltage VFD driven motor system is described
below with reference to FIG. 1. The neutral point N 26 of the DC
bus 20 is grounded to protect the transistor switches from
potential voltage spikes that would cause insulation degradation
and component failure. The heat sink plate of transistors in the
inverter bridge is also grounded, however, the ground connection is
not shown in FIG. 1. FIG. 2A illustrates grounding of the inverter
bridge transistor module via the heat sink plate 126 and is
described below in greater detail. Again referencing FIG. 1, three
phase cables 30 are connected at one end to the output terminals 52
of the VFD 50. Cables 30 have an inherent capacitance per unit
length. The total cable capacitance is shown as C.sub.C 32. These
cables feed the motor M 40, which also has capacitance due to
windings, shown as C.sub.M 42, and motor impedance shown as Z.sub.M
44.
FIG. 3A is a schematic circuit 300 representation of a conventional
VFD driven motor system, for example the drive system shown in FIG.
1. Switch S 60 represents the voltage transitions output from the
VFD 50. Upon closing of switch S 60, a voltage transition, from
grounded neutral to the positive 22 or negative 23 potential on the
DC bus 20, output from the VFD 50 is imposed upon the circuit 300.
Ground leakage current I.sub.GND 200 flows freely in the ground
connection with the voltage transitions due to the motor
capacitance C.sub.M 42 and the cable capacitance C.sub.C 32.
Inverter bridge capacitance, C.sub.IB 62, is connected from the
neutral point N 26 to equipment ground PE 70 and to true earth
ground TE 80 in parallel with the short circuit of the neutral
point N 26 connection to true earth ground TE 80. In this
conventional configuration because C.sub.IB 62 is in parallel with
the short circuit connection to ground, C.sub.IB 62 contributes
negligibly to ground leakage current.
Because of their high performance and lower power consumption, VFDs
are desirable in a variety of demanding applications, to include
fan and pump loads. However, use of VFDs in medium voltage
applications can be complicated if low ground leakage current is
necessary. Low ground leakage current can be necessary in
potentially explosive environments or in environments requiring
reduced electromagnetic interference (EMI). High frequency ground
leakage currents, up to the MHz range can lead to EMI, for example
in radio receivers, computers, bar code systems, and vision
systems.
One example of an application requiring low ground leakage current
is underground mining; the underground mining environment has
unique requirements and safety standards. Underground mining motors
are preferably in the medium voltage range (between 690 V and 15
kV) and are typically driven at 4,160 V. A conventional medium
voltage VFD providing a 4,160 V output can yield a ground leakage
current I.sub.GND 200 in excess of ten amps, which flows from the
VFD 50 to the motor M 40 in the grounding wire. While using a
medium voltage motor facilitates the use of smaller cables, the
maximum permitted drive to motor ground wire leakage current
I.sub.GND 200 can be below 1 Amp.
Unlike conventional AC sinusoidal motor drives, VFDs output voltage
transitions on the time order of microseconds. Consequently, large
ground leakage currents are induced due to capacitances C.sub.M and
C.sub.C, inherent in a VFD driven motor system, even at relatively
low voltages, for example 690 volts. Referring to FIG. 3A,
disconnecting the neutral point N 26 of the DC bus 20 from TE
ground 80 appears to be a viable means of reducing ground leakage
current. A schematic representation of disconnecting the neutral
point N 26 from ground in a conventional VFD system 302 is shown in
FIG. 3B. As shown in FIG. 3B, disconnecting the neutral point N 26
of the DC bus from TE ground 80 changes the circuit model for
ground current leakage current, I.sub.GND 202. Inverter bridge
capacitance, C.sub.IB 62, is now in series with the parallel
combination of cable capacitance C.sub.C 32 and motor capacitance
C.sub.M 42. This results in higher impedance for the ground leakage
current due to the decrease in total system capacitance. However,
disconnection of the neutral point N 26 from TE ground 80 leaves
transistors S.sub.1-S.sub.12 in the inverter bridge susceptible to
voltage spikes.
Disconnecting the neutral point N 26 of the DC bus from TE ground
80, leaves the transistors floating relative to the neutral point N
26 of the DC bus. Voltage spikes at full DC bus potential can be
applied across the transistors in the inverter bridge. Referring to
FIG. 2A, these voltage spikes are transmitted between the
transistors' semiconductor substrate 122 and the transistors' heat
sink plate 126 across thin insulator 124.
For lower drive voltages, available transistors rated above the
difference between the positive and negative DC bus can be employed
in a VFD system having the neutral point of the DC bus disconnected
from TE ground and left floating. This configuration is
successfully employed for example in SMC's Microdrive 2,300 V
model.sup.1. However, when higher VFD voltage output is needed or
desired and when transistors rated at the full DC bus potential are
not practical, protecting transistors from full DC bus potential
spikes is necessary to prevent reduced component life and component
failure. Multiple challenges exist for VFD drive applications. One
challenge, for example, is to reduce leakage ground current while
protecting the VFD, in particular the inverter bridge. Another
challenge is to reduce ground leakage current as much as possible.
.sup.1 VFD, Microdrive, 2,300V model, SMC Electrical Products,
2003.
For other applications, the challenge is to provide a reliable VFD
system for motors rated at greater than 4160 V. For, example for a
motor rated at greater than 4160 V, a VFD providing an output of
6.9 kV output is desirable. However, presently available
transistors to build a VFD with a 6.9 kV output are susceptible to
compromised transistor insulation and impending component failure.
A DC bus rated at 11.5 kV is needed to achieve a VFD output of 6.9
kV. Inverter bridge transistors are available at an insulation
rating of 5,100 V. Even when the neutral point of the DC bus is
grounded, the transistor module insulation 124 (FIG. 2A) breakdown
voltage (5,100 V) is less than half the potential on the DC bus
(11.5 kV). Yet another challenge in VFD systems is to protect the
inverter bridge comprising available transistors connected in
series to provide VFD output voltages greater than 4160 V when
transistors are rated at less than half of the DC bus voltage.
SUMMARY OF THE INVENTION
The present invention provides decreased ground leakage current in
a VFD driven motor system, while protecting the inverter
bridge.
It is an object of the present invention to reduce ground leakage
current in a VFD driven motor system.
It is another object of the present invention to reduce the ground
leakage current by floating the neutral point of the DC bus.
It is another object of the present invention to float the neutral
point of the DC bus while protecting the VFD from component failure
due to voltage spikes.
It is another object of the present invention to increase the
impedance for ground leakage currents.
It is another object of the present invention to further reduce
ground leakage current in a medium voltage VFD driven motor system
without decreasing system capacitance to ground.
It is another object of the present invention to decrease the total
capacitance to ground of a VFD motor driven system.
It is another object of the present invention to decrease the total
capacitance to ground of a VFD motor driven system by means of a
high dielectric strength and low dielectric constant plate disposed
between the VFD transistor module heat sink plate and a grounded
cooling plate.
Another object of the present invention is to improve inverter
bridge reliability and component life in a VFD system having
transistors rated at less than half of the full DC bus potential by
means of additional effective insulation when the neutral point of
the DC bus in the VFD is grounded.
Exemplary embodiments of the present invention can be used in low,
medium, and high voltage drive applications.
In accordance with the objects of the present invention, in an
apparatus according to an exemplary embodiment of the present
invention, the neutral point of the DC bus is floating,
disconnected from ground.
In an apparatus according to another exemplary embodiment, an
electrically insulating plate having high thermal conductivity,
high dielectric strength, and a low dielectric constant is
thermally and electrically connected between the transistor
semiconductor substrate and the cooling plate.
In an apparatus according to another exemplary embodiment, a common
mode filter is installed at the output of the VFD.
A method in accordance with an embodiment of the present invention
comprises floating the neutral point of the DC bus and increasing
the impedance of the ground leakage path by means of a dielectric
substrate.
Another method in accordance with an embodiment of the present
invention comprises increasing the impedance of the ground leakage
path by means of a dielectric substrate and also increasing the
dielectric strength of the transistor module of the VFD to greater
than the full DC bus voltage.
Another method in accordance with another embodiment of the present
invention comprises floating the neutral point of the DC bus,
increasing the impedance of the ground leakage path by means of a
dielectric substrate, and installing a common mode filter across
the three phase drive cables in a VFD system.
Other objects and advantages of the present invention will become
apparent to one skilled in the art from the following description
in view of the accompanying drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 shows a conventional VFD driven motor system.
FIG. 2A shows grounding of an inverter bridge across the transistor
module's insulating plate and resulting inverter bridge
capacitance, according to a conventional VFD system.
FIG. 2B shows the series connection of the inverter bridge to
ground across the transistor module's insulating plate, the
module's heat sink plate, and the electrically insulating, high
dielectric strength, low dielectric constant, and high thermal
conductivity plate according to an embodiment of the present
invention.
FIG. 3A shows a schematic representation of a ground current loop
in a conventional VFD system, as shown for example in FIG. 1, to
include the capacitance of the inverter bridge.
FIG. 3B shows a schematic representation of a VFD system having a
floating neutral point on the DC bus in the absence of the present
invention.
FIG. 4 shows a schematic representation of a ground current loop in
a VFD system implementing an exemplary embodiment of the present
invention, showing the capacitance of the transistor module and the
capacitance of the low dielectric constant insulating plate.
FIG. 5 shows another exemplary embodiment of the present invention
comprising a common mode filter installed on the three phase drive
cables.
FIG. 6 shows an exemplary embodiment of a mounting means which
enables electrical insulation and thermal conduction between the
VFD power semiconductor module and the grounded cooling plate and
secures the VFD power semiconductor module to the insulating plate
and secures the insulating plate to the grounded cooling plate.
DETAILED DESCRIPTION OF THE DRAWINGS
The present invention reduces ground leakage current by allowing
the neutral point N of the DC bus to float without exposing the
transistors in the inverter bridge to excessive voltage spikes.
Turning first to FIG. 2A, the conventional grounding of an inverter
bridge in a transistor module is shown. Grounding of the inverter
bridge is via the transistor module's heat sink plate 126. A
capacitance C.sub.IB 62 is formed between the semiconductor
substrate 122 and the heat sink plate 126 across the thin
insulating plate 124. The semiconductor substrate 122, the thin
insulating plate 124, and the heat sink plate 126 collectively form
a conventional VFD power semiconductor module, also commonly
referred to as a VFD transistor module. The heat sink plate 126 is
then mounted on and electrically connected to the grounded cooling
plate 130.
FIG. 2B shows grounding of an inverter bridge in accordance with an
exemplary embodiment of the present invention. The transistor
module's semiconductor substrate 122, insulating plate 124 and heat
sink plate 126 are connected together as in the conventional
grounding means of FIG. 2A. However, according to an exemplary
embodiment, an electrical insulator plate P 175 having high
dielectric strength, low dielectric constant, and high thermal
conductivity is mounted between the heat sink plate 126 and the
grounded cooling plate 130. The heat sink plate is typically made
of aluminum silicon carbide. However, the heat sink plate 126 need
not be made of aluminum silicon carbide; a material providing good
electrical conduction and thermal conduction would be an adequate
substitute.
The inverter bridge semiconductor substrate 122 is thermally
connected to and electrically insulated from the cooling plate 130
via plate P 175. Because the dielectric constant of insulating
plate P is low, a small capacitance C.sub.P 176 is formed between
the transistor module's heat sink plate 126 and the cooling plate
130. This capacitance is smaller than and is in series with the
transistors internal capacitor C.sub.IB 62 (FIGS. 2A, 2B, and 4).
Capacitance C.sub.IB 62 is a byproduct of the capacitive coupling
between the semiconductor substrate surface 122 and the transistor
heat sink plate 126, which are insulated from each other with the
thin insulator 124, as shown in FIG. 2A.
Referring to FIG. 3A, in the conventional inverter bridge,
grounding the neutral point N 26 of the DC bus 20 and grounding one
end of C.sub.IB 62 to the same TE ground 80 creates a path to the
DC Bus for displacement currents generated at every pulse
transition. This grounding prevents one transistor from affecting
another transistor in the inverter bridge. More particularly,
removal of the neutral point N 26 grounding of the DC bus 20 causes
one side of C.sub.IB 62 to be floating relative to the neutral
point N 26 of the DC bus, as shown in FIG. 3B, which allows
displacement current crosstalk between transistors in the inverter
bridge via respective transistor capacitances, C.sub.IB 62, which
are interconnected via cooling plate 130. This crosstalk can cause
excessive voltage spikes between the transistor module's
semiconductor substrate 122 and its heat sink plate 126 that can
lead to component failure.
It is known by those skilled in the art that modular or isolated
base high voltage IGBT transistors require the neutral point of the
DC bus to be grounded. Such grounding ensures that the maximum
voltage between the transistor terminals and the cooling plate is
no more than half of the maximum DC bus voltage. If the neutral
point is disconnected from ground, the capacitor, which is formed
between the transistor module's semiconductor substrate and its
heat sink plate, C.sub.IB is still grounded on the cooling plate
130 side. The inverter bridge capacitance is now, floating relative
to the DC bus positive and negative voltages 22/23 and is
electrically connected to the other transistors' internal
capacitances, C.sub.IB 62, in the inverter bridge via the grounded
cooling plate 130. When the transistors which are attached to the
positive voltage of the DC bus are turned off, the semiconductor
substrate is connected directly to the positive voltage of the DC
bus. Then, when the transistors connected to the negative voltage
are turned on, this causes internal transistor capacitances,
C.sub.IB 62, to be charged at the negative voltage potential of the
DC bus. Because one side of all of the transistors' respective
internal capacitors, C.sub.IB 62, are connected together, the
semiconductor substrate of the transistors connected to the
positive voltage are subjected to the full DC bus voltage. In the
typical case, the full DC bus voltage is substantially higher than
the transistor's insulation voltage rating and damage to the
transistor occurs.
FIG. 4 shows a schematic representation 304 of an exemplary
embodiment of a VFD system according to the present invention
incorporating an insulating plate P 175, as shown for example in
FIG. 2B. The insulator plate capacitance C.sub.P 176 is now in
series with C.sub.IB 62. The insulator plate capacitance, C.sub.P
176 is connected to the TE grounded cooling plate 130. The series
capacitance of C.sub.IB and C.sub.P provides higher impedance for
the return path to the voltage source V 140, decreasing ground
leakage current I.sub.GND 204. The insulating plate P 175 (FIG. 2B)
also serves to increase the insulation strength between the cooling
plate and the power semiconductor. Total system capacitance
C.sub.SYS is decreased as described by equations 1 and 2
corresponding to the circuit shown in FIG. 4. First, in equation 1,
we define the capacitance of the VFD, C.sub.VFD.
##EQU00001##
The decrease in total system capacitance C.sub.SYS in turn reduces
the ground leakage current I.sub.GND 204 from the voltage
transitions at the output of the VFD according to equation 3.
'dd ##EQU00002##
The following experimental data shown in Tables 1 and 2 was
obtained in the presence and absence of an exemplary embodiment of
the present invention and confirms the effectiveness thereof. Table
1 summarizes the data obtained under control conditions. The VFD
module is a 4160 V output Microdrive (SMC Electrical Products, U.S.
Pat. No. 6,822,866). The motor is a 500 HP induction motor rated at
4000 V or less. Ground current measurements were made for three
phase shielded cable lengths of 30, 250, and 1300 feet. Ground
current was continuously measured at the output terminals of the
VFD and at the motor. The VFD was switching at 1 kHz, and the motor
speed was maintained at 30 percent. Control measurements could not
be made with the neutral point N of the DC bus disconnected from
ground and floating in the absence of an insulating plate P 175, as
depicted in FIG. 3B. Disconnecting the neutral point N from ground
left the transistors of the inverter bridge only protected by thin
insulator 124, resulting in component failure due to crosstalk, as
expected and as discussed above. A 1.5 .mu.F capacitor was
connected in series from the neutral point N to ground and in
parallel with C.sub.IB, to serve as a very high impedance path,
providing more protection than an open circuit. The 1.5 .mu.F
capacitor provides a high impedance path for the neutral point to
ground without sacrificing the inverter bridge. Table 1 summarizes
the control data obtained without the insulating plate P 175.
Current values in Tables 1-3 are RMS.
TABLE-US-00001 TABLE 1 GROUND CURRENT MEASUREMENTS FOR 30, 250, and
1300 ft. CABLES (Amps) Without Boron Nitride Plate 30' 250' 1300'
@Drive 4.7 8.0 38.3 @Motor 2.3 2.0 3.7
Table 2 summarizes experimental data obtained using an exemplary
embodiment of the present invention. The test conditions were the
same as those of the control conditions, above, with the following
test modifications. An insulating plate P 175 was installed between
the heat sink plate 126 and the cooling plate 130 (as shown in
FIGS. 2B and 6). The neutral point N of the DC bus was disconnected
from ground and floating. In this exemplary test embodiment, the
insulating plate P is ceramic made from boron nitride. FIG. 4 is a
schematic representation of the test conditions summarized in Table
2.
TABLE-US-00002 TABLE 2 GROUND CURRENT MEASUREMENTS FOR 30, 250, and
1300 ft. CABLES (Amps) WITH Boron Nitride Plate 30' 250' 1300'
@Drive 0.7 1.1 5.5 @Motor 0.5 0.3 0.4
Additional experimental measurements were made for the system
according to another exemplary embodiment, comprising a common-mode
filter (CMF 150), shown for example in FIG. 5. The CMF 150,
transformer and resistor ballast, is installed across the three
phase cables connecting the VFD output to the motor. Acquisition of
the data below in Table 3 was made with a 500 HP induction motor
running at 50 percent of full speed. All other test conditions were
the same as those employed to acquire the test data summarized in
Table 2. The VFD was switching at frequency of 1 kHz. Ground
current measurements were made at the output terminals of the VFD
and at the motor. Current values shown are RMS, as above.
TABLE-US-00003 TABLE 3 GROUND CURRENT MEASUREMENTS 250 ft CABLES
(Amps) WITH Boron Nitride Plate No CMF CMF @Drive 5.2 0.05 @Motor
0.8 0.05
As seen from the data in Table 3, above, ground leakage current is
reduced to a negligible amount, 50 mA, with installation of the
boron nitride ceramic plate and the CMF according to another
exemplary embodiment of the present invention.
While boron nitride plates were used in the exemplary embodiments
for experimental data acquisition described above, other oxide and
nitride materials or other insulating substances that have the
desired dielectric and thermal properties described according to
the present invention can be used. For example, synthetic diamond
plates can be used which have the desired dielectric properties and
excellent thermal conduction.
Installation of the insulation plate 175, as shown in FIG. 2B, also
provides another object of the present invention, which is to
enable the use of transistors rated at less than half of the full
DC bus potential in building a reliable VFD system with the neutral
point of the DC bus grounded. For example, for a motor rated at
greater than 4160 V, a VFD providing an output voltage greater than
4160 V, for example 6.9 kV, is desirable. DC buses rated at 11.5 kV
are readily available for use in VFDs and are adequate to provide a
VFD output of 6.9 kV. Transistors to build the inverter bridge are
readily available at a rating of 5,100 V. Even when the neutral
point is grounded, the transistor insulation (i.e., element 124 in
FIG. 2A) breakdown voltage (5100 V) is less than half the full
potential on the DC bus 11.5 kV. Installation of insulating plate P
175, as shown in FIG. 2B, with sufficient dielectric strength as
described above, increases the effective insulation of the inverter
bridge transistors analogous to the effect of C.sub.P in series
with C.sub.IB versus C.sub.IB alone. The increased insulation
improves component life and system reliability. Therefore,
installation of the insulation plate P 175 enables multiple
transistors to be connected in series (shown for example in FIGS. 1
and 5, S1-S12) and cooled by a common cooling plate 130 (shown for
example in FIG. 2B) to achieve a higher output voltage VFD system
at low cost using presently available transistors, rated at below
half the full DC bus voltage, while maintaining the benefits of a
single grounded cooling plate.
FIG. 6 shows an exemplary embodiment of a mounting means which
enables electrical insulation and thermal conduction between the
VFD power semiconductor module 50 and the grounded cooling plate
130 and secures the power semiconductor to the insulating plate 175
and secures the insulating plate to the grounded cooling plate 130.
The VFD power semiconductor module 50 (also shown in FIGS. 2A and
2B) is securely mounted on the insulating plate 175, which is
securely mounted on the grounded cooling plate 130 by means of L
shaped steel brackets 180. Brackets 180 are fastened directly to
the cooling plate 130 on one end and secure the VFD power
semiconductor module via set screws 182 and ceramic pellets 184 on
the other end.
FIG. 6 shows only one exemplary embodiment of many possible means
for securing the VFD power semiconductor module and the insulation
plate to the cooling plate. The bracket can be made from any
material having sufficient strength to support the clamping forces
required by the transistor module specifications. One ordinarily
skilled in the art will readily appreciate the various ways of
physically securing the VFD power semiconductor module to the
insulating plate and the insulating plate to the cooling plate
while permitting the electrical insulation and thermal conduction
capacities of the insulation plate to be realized.
In summary, reduction of ground leakage current in a VFD system is
desirable for multiple reasons in numerous application
environments. An electrical insulator plate having high dielectric
strength, low dielectric constant, and high thermal conductivity
mounted between the VFD power semiconductor module and the grounded
cooling plate is an effective means of reducing system capacitance
thereby reducing ground leakage currents induced with the high
frequency voltage shifts of a VFD. Installation of the insulator
plate described above protects the transistors in the inverter
bridge from insulation breakdown by increasing the insulation
between the VFD power semiconductor module and ground.
While the present invention has been particularly shown and
described according to exemplary embodiments herein, it will be
understood by those skilled in the art that various changes can be
made in form or detail without departing from the spirit and scope
of the invention as defined by the following claims.
* * * * *